I'm starting a mobile campaign and have been told repeatedly to not offer in the US. But what about Australia and the UK? I think I saw a thread about how both countries have a lot of affiliates, which makes me think these affiliates would naturally first try to advertise in their own countries, which would make them competitive environments.
US is generally very competitive and can be a hard place to start on a low budget.
Obviously each country has pros and cons - some have more traffic than others. Some are more expensive than others.
If you want to avoid competition, go for non english speaking countries - most affiliates are too lazy to translate their landing pages and banners.